The present invention relates to semiconductor integrated circuits (ICs), and more particularly to assemblies of multiple ICs.
FIG. 1 illustrates a typical assembly with multiple ICs 110 connected to a printed circuit board (PCB) 114. Each IC 110 is a die initially fabricated with other die as part of a larger wafer but then separated from the wafer. Oftentimes, die 110 are not attached to the PCB directly because PCB contact pads 114C cannot be spaced as closely from each other as the die's contact pads 110C due to coarser PCB fabrication technologies. Therefore, the die are attached to the PCB through an interposer (ITP) 120; the interposer's top contact pads 120C.T match the die's contact pads 110C, and the interposer's bottom contact pads 120C.B match the PCB contact pads 114C. The interposer thus provides a contact redistribution function. Also, in many assemblies, the die's coefficient of thermal expansion (CTE) is different from the PCB, and the interposer relieves thermal stresses due to the CTE mismatch between the die and the PCB.
The die-to-interposer and interposer-to-PCB attachments are shown at 126, and they can be solder, conductive or anisotropic adhesive, or can be formed by direct diffusion bonding without additional solder or adhesive materials. PCB 114 includes conductive interconnects 114L which interconnect the PCB contact pads 114C in a desired pattern.
Interposer 120 is manufactured based on a substrate 120S, possibly silicon, glass, or some other material. Conductive vias 130 pass through the substrate and terminate at contact pads 120C.T. (Vias 130 can be electrically insulated from substrate 120S by dielectric 134 if the substrate 120S is not dielectric.) On top of substrate 120S, a redistribution layer (RDL) 140 provides the top contact pads 120C.T and also provides conductive lines 140L that connect the vias 130 to contact pads 120C.T (RDL lines 140L may also connect the top contact pads 120C.T to each other, and/or connect the vias 130 to each other.) Lines 140L are electrically insulated from each other and, if needed, from substrate 120S, by RDL dielectric 140D.
Underfill (“UF”) 150 is introduced between the die 110 and interposer 120 to glue the die to the interposer around the attachments 126. Underfills reduce the thermal stresses (mechanical stresses generated by thermal expansion) on connections 126, which is especially important if the die CTE in the XY plane (the plane of interposer 120) does not match the interposer. A typical underfill is a plastic organic polymer (e.g. cured epoxy resin), often with additives (e.g. fire retardants), possibly with hard-particle fillers (silica, alumina, or others) used to reduce the underfill CTE and possibly adjust other parameters of interest, and to lower the cost. An encapsulant 160 covers the die to protect them from contaminants and to mechanically strengthen the structure. The encapsulant can be organic polymeric material (possibly with additives, including hard-particle fillers), the same or similar to the underfill materials described above, and can be deposited in a flowable form (liquid or semi-solid, e.g. by molding or without a mold), or by chemical vapor deposition (CVD). Underfill 162 (possibly but not necessarily the same material as 150) can glue the interposer 120 to PCB 114 around the corresponding connections 126.
In many applications, the interposer substrate 120S should be thin to reduce the assembly size and the length of vias 130. However, a thin substrate is fragile and, in addition, is easily warped by internal and external stresses, e.g. thermal stresses. Therefore, in some manufacturing processes, the interposer fabrication starts with a thick substrate 120S, and the substrate is thinned at a later stage. For example, in FIG. 2A, the substrate 120S is initially thick, and the vias 130 penetrate the substrate only partially. Substrate 120S is thinned only after formation of RDL 140 (FIG. 2B); the thinning exposes the vias 130 on the bottom. Further, before the substrate is thinned, it is strengthened by a temporary carrier wafer 210 attached to the interposer's top surface; the carrier wafer provides mechanical strength, reduces warpage, and improves heat dissipation. Mechanical strength is particularly important if the interposer thinning involves mechanical processes such as grinding, lapping, chemical-mechanical polishing (CMP), etc.
Carrier wafer 210 is later removed to allow die attachment to the top of the interposer. Alternative processes are desirable.